Generally, a polymer electrolyte membrane fuel cell (PEMFC) is used as a fuel cell for automobiles. In order for this polymer electrolyte membrane fuel cell to normally exhibit a high-power performance of several tens of kilowatts (kW) or more under various operation conditions, it needs to be stably operated in a wide current density range.
In the polymer electrolyte membrane fuel cell, an electrochemical reaction for generating electricity takes place in a membrane-electrode assembly (MEA) including a perfluoro-sulfonic acid membrane and an electrode pair of anode and cathode. In the membrane-electrode assembly, hydrogen supplied to the anode (oxidation electrode) is dissociated into hydrogen ions (protons) and electrons. Then, the hydrogen ions are transferred to the cathode (reduction electrode) through the membrane, the electrons are transferred to the cathode through an external circuit, and the hydrogen ions and electrons transferred to the cathode react with oxygen molecules to generate electricity, heat, and byproduct water (H2O). In this case, when a suitable amount of water is produced during the electrochemical reaction in the fuel cell, the humidity of the membrane-electrode assembly can be maintained, but when an excess amount of water is produced, a water flooding phenomenon occurs at a high current density if it is not properly removed, and the flooded water inhibits reaction gases from being efficiently supplied into the fuel cell, thereby increasing a voltage loss.
In the electrochemical reaction of the fuel cell, when hydrogen ions are transferred from the anode to the cathode through the membrane, they are bonded with water molecules in the form of hydronium ions (H3O+) to drag water molecules. This phenomenon is referred to as “electro-osmotic drag (EOD)”. When the amount of water accumulated in the cathode increases, a part of water is reversely transferred from the cathode to the anode, which is referred to as “back diffusion (BD)”. Therefore, in order for the fuel cell to obtain an excellent cell performance, these water transfer phenomena need to be clearly understood, and the water existing in the fuel cell needs to be efficiently used.
Generally, a fuel cell vehicle uses a fuel cell stack which is a laminate of several hundreds of unit cells. This unit cell includes a membrane-electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, and a separator. The gas diffusion layer must be attached to the anode and cathode of the MEA. In this case, in order to improve the handling property of the MEA, high stiffness is needed. Further, the MEA receives compression pressure for a long time through a gasket made of elastic rubber, and thus, this gasket needs to maintain its form without being torn or deformed even under a compression condition. For this purpose, a solid phase film-type subgasket is generally attached to the periphery of a membrane by thermal lamination. In the subgasket lamination process, in order to improve the adhesion between the membrane and the subgasket, an adhesive may be used together with heat, but may not be used according to material properties of the subgasket and a required adhesivity. As such, when the MEA provided with the subgasket is used, the fuel cell stack can be used for a long time even though it is laminated with several hundreds of MEAs.
In the above conventional MEA structure, in order to enhance the attachment of the subgasket to the membrane, the membrane is extended to the outer wall of a fuel cell and then attached to the subgasket in addition to the active area in which the anode and cathode used in the electrochemical reaction of the fuel cell are attached to the membrane. However, in this MEA structure, a large expensive membrane needs to be used, and particularly, undesired water diffusion occurs. Thus, water to be used for a fuel cell reaction is lost, and corrosion of other fuel stack components made of metal materials is accelerated, thereby greatly deteriorating operation stability of an automobile.
As another conventional technology, there is a proposed method of attaching a subgasket to an MEA using injection molding, rather than thermal lamination. However, in this method, the MEA is deformed or contaminated during the injection molding of the subgasket. Further, a complicated multi-step process needs to be used in order to overcome this problem. Therefore, this method is also problematic in that the subgasket attaching process becomes complicated compared to when thermal lamination is used, and in that the productivity thereof is lowered.
It is to be understood that the foregoing description is provided to merely aid the understanding of the present disclosure, and does not mean that the present disclosure falls under the purview of the related art which was already known to those skilled in the art.